1. Field of the Invention
The present invention relates to a vehicle headlamp according to the preamble of claim 1. Such a vehicle headlamp is disclosed in utility model G 90 00 395.
2. Description of Related Art
Specific location-dependent illuminances are required for vehicle headlamps in the SAE legal sphere. These are prescribed separately for low beam light and high beam light with the aid of numerous measurement points. Alongside prescribed illuminances of the low beam light impinging on the road, that is to say below the so-called light/dark boundary, there is also a need to meet special requirements above the light/dark boundary. This area of the light distribution is denoted as overhead or sign light area, the latter term being derived from the visibility of traffic signs. The legal measuring points of the so-called overhead light extend up to four degrees above a line that marks the horizon, and are characterized by minimum values and maximum values of the lighting intensities permissible there.
In the case of reflection systems, the measuring points can be very easily catered for with minimum values by a suitable shaping of the reflector. Projection systems with a low beam light function light the area above the light/dark boundary only very weakly, because of their optical principle. Consequently, it is necessary in the case of these projection systems to take measures to meet the minimum values of the intensities in this area. This can be achieved, on the one hand, by means of additional, reflecting components in the projection system such as, for example, reflecting diaphragms. Thus, it is known to make use of additional sheets that are arranged horizontally in the beam path and are strongly reflecting at high incidence angles. Said additional sheets constitute an additional component and therefore undesirably cause higher costs and a greater system complexity.
Other solutions provide a local change in geometry within the projection lens that exerts a suitable prismatic effect on a portion of the light flux penetrating through the lens.
The vehicle headlamp disclosed in utility model G 90 00 395.0 has an imaging optics with a lens and a diffusion lens that serves well as transparent cover of the vehicle headlamp. In order to suppress a color fringe, the light exit boundary surface of the lens is divided horizontally into an upper aspheric segment and a lower aspheric segment. At the transition between the two segments, the lens has a part of a horizontally arranged convex cylindrical lens. This cylindrical lens upwardly deflects the portion of the lens light flux that penetrates the cylindrical lens into a defined angular range such that this deflected light flux lights the overhead area.
In the case of another configuration, a sector of a horizontally arranged concave cylindrical lens is integrally formed on the light entrance boundary surface of the diffusion lens. This cylindrical lens likewise directs upward the partial light bundle traversing it. A further configuration specifies that it is also possible to provide a plurality of cylindrical lenses one above another vertically on the diffusion lens. The overall aim in the case of the known vehicle headlamp is to enable the upwardly deflected partial light bundle to fulfill legally required light values for fog lamps above the light/dark boundary.
Further examples of local deformations of the lens surface of the projection lens which, owing to their additional prismatic action, deflect light into measuring points within an overhead area are disclosed in the publications DE 10 2004 024 107 A1 and U.S. Pat. No. 6,971,778. DE 10 2004 024 107 A2 discloses for this purpose a lens with a cylindrical section running horizontally in the middle. U.S. Pat. No. 6,971,778 exhibits in this context a local depression in the lower region of the lens. Also known are geometric surface modifications which, however, do not primarily aim at producing defined overhead intensity values. These include, for example, lenses with horizontally and obliquely running corrugated structures. Such a lens is disclosed in DE 40 31 352 A1.
A great disadvantage of the known solutions, which operate with a local deformation of the lens surface, resides in the fact that they all react sensitively to even a slight change in the light distribution. Consequently, given the occurrence of a series of manufacturing tolerances, markedly altered illuminances can occur in the range of the sign-light measured values. The consequence is that it cannot be ensured that an overhead solution can be used on another projection system (that is to say in the event of a change in light distribution). This means, in turn, that it is necessary as a rule to find for each projection module new solutions to the production of the prescribed overhead lighting intensity values.
It is also disadvantageous that the required prismatic action always constitutes a clearly visible incursion into the lens surface that marks its appearance. In the case of previously disclosed solutions, this is partly felt as unaesthetic, or the local deformations are perceived as defects in the projection lens. Thus, various overhead solutions are perceived as conchoidal fractures (instances of splitting), glass defects such as inclusions and bubbles or visible jumps in the lens (for example in the case of horizontally arranged cylindrical lens sectors)—all of these being typical of glass.